Explain why aluminium fluoride and aluminium oxide are ionic whereas aluminium chloride is covalent.
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keep a long story short, aluminium chloride exhibits covalent character because
The aluminium ion has high charge densityThe electron cloud of the chloride is distortable (JC term, polarizable)The combination of the 2 factors above leads to some degree of electron sharing between the positive aluminium ion and the negative chloride ion and hence the covalent character.
In general, in order for an ionic compound to have covalent character (oxymoronic, I know.. but.. ) the positive charge of the cation must be so strong that it causes the electron cloud of the anion to be distorted (fancy JC term, polarized) such that electrons become drawn into the space between the positive cation and the negative anion. Hence, the electrons are said to be “shared” and the ionic compound is said to exhibit some covalent character. (See diagrams below)

Scenario 1: Large cation = lower charge density, not enough to distort the electron cloud of the anion significantly.
Scenario 2: As the charge density of the cation increases (represented by smaller cation), the attractive power of the cation increases and the electron cloud of the anion becomes increasingly distorted.

Scenario 3: When the charge density of the cation is high (represented by a small cation), the attractive power of the cation is strong enough to distort the electron cloud of the anion such that some electrons are “shared” between the 2 ions.
Let’s take a deeper look at the 2 factors described briefly above:
“Strength” of cation (JC term, polarizing power of cation)Ease of distortion of electron cloud of anion (JC term, polarizability of anion)
“Strength” of cation
“Strength” of cation is related to its charge density, which can be loosely defined as charge divided by volume of cation. The larger the charge density, the more “concentrated” the positive charge (the cation becomes more “powerful” so to speak), the more likely it is able to attract electrons from the anion into the space between the cation and the anion, thereby creating “electron sharing” and hence covalent character
The aluminium ion has high charge densityThe electron cloud of the chloride is distortable (JC term, polarizable)The combination of the 2 factors above leads to some degree of electron sharing between the positive aluminium ion and the negative chloride ion and hence the covalent character.
In general, in order for an ionic compound to have covalent character (oxymoronic, I know.. but.. ) the positive charge of the cation must be so strong that it causes the electron cloud of the anion to be distorted (fancy JC term, polarized) such that electrons become drawn into the space between the positive cation and the negative anion. Hence, the electrons are said to be “shared” and the ionic compound is said to exhibit some covalent character. (See diagrams below)

Scenario 1: Large cation = lower charge density, not enough to distort the electron cloud of the anion significantly.
Scenario 2: As the charge density of the cation increases (represented by smaller cation), the attractive power of the cation increases and the electron cloud of the anion becomes increasingly distorted.

Scenario 3: When the charge density of the cation is high (represented by a small cation), the attractive power of the cation is strong enough to distort the electron cloud of the anion such that some electrons are “shared” between the 2 ions.
Let’s take a deeper look at the 2 factors described briefly above:
“Strength” of cation (JC term, polarizing power of cation)Ease of distortion of electron cloud of anion (JC term, polarizability of anion)
“Strength” of cation
“Strength” of cation is related to its charge density, which can be loosely defined as charge divided by volume of cation. The larger the charge density, the more “concentrated” the positive charge (the cation becomes more “powerful” so to speak), the more likely it is able to attract electrons from the anion into the space between the cation and the anion, thereby creating “electron sharing” and hence covalent character
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